5 year old 11 speed mechanical Ultegra group - always worked fine and recently moved to a newer frame. System and rear derailleur shifts perfectly and everything seems fine...until...

When descending over a rougher road (chip seal) and not pedaling, I hear a chatter that sounds just like chain slap. But I can't see or find any physical evidence that that is what it is. And it goes away when I continue to pedal. For a moment, I thought it might be the rear hub but that's been checked and cleared.

I'm now thinking B screw adjustment but if anything, its adjusted a hair too far from the cogs as opposed to too close.

Thoughts?

Thanks!

55/Rad